ADE7758: Compute active power, power factor and how to calculate in

Document created by analog-archivist on Feb 23, 2016
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Q

1) - to put on in line accumulation mode and to read the value of energy has
accumulated
during the number of planned cycles, then to divide for the time of
accumulation.
Then for every phase..

P = EP / time_acc [W]

Q = EQ / time_acc [VAR]

S = ES / time_acc [VA]

2) - to plan the device so that to have in the WREG the instant samples of p
(n), q (n), s (n)
for every phase.

To every interrupt WFSM the uP reads the sample from the WFORM register,
the powers for every phase are calculated then as:

M = number champions

P = 1/M * sum (p (n)) n = 0... M-1

Q = 1/M * sum (q (n)) n = 0... M-1

S = 1/M * sum (s (n)) n = 0... M-1

Are the 2 procedures correct ?

Are there other methods ?

3) Power factor PF:

For every phase I must be able to draw the value of the PF with
his sign (capacitive or inductive). I have then to also draw the total PF.
FAQ-AN639 give it is said to the registers LAENERGY / LVAENERGY this gives me
the PFtot without sign,
in 7758 I would owe therefore:

To put me in LINE CYCLE ACCUMULATION mode To plan the number of cycles to have
the interrupt.
For every phase to calculate:

PF_A = AWATTHR / AVARHR * sgn (QA)

PF_B = BWATTHR / BVARHR * sgn (QB)

PF_C = CWATTHR / CVARHR * sgn (QC)

sgn (Q) = sign of the reactive energy drawn by the bit of sign of the regs To,
B, C VAHR

PFtot = (Etot / Stot) * sgn (Qtot)
or from the data of power drawn above to calculate the PF as:

PF = (P / S) * sgn (Q)

this last is the method that I have adopted and it works.
Are there other methods to make the measure of the PF ?

A

1) how to compute the active power: the active power is the dc component of the
instantaneous power signal p(t) in equation 14 in the data sheet.This component
is obtained after LPF2 filter and can be read with the waveform register when
WAVSEL bits of WAVMODE register are set to Active Power Multiplier Output. Of
course, this reading must then be transformed in Watts. An alternative method
is presented in AN-639 for ADE7754.

2) how to compute the power factor: the active power computed above is divided
by the apparent power (the product between voltage rms value and current rms
value).
3) how to calculate In: I believe that because the meter will work for 50/60Hz
inputs, 10 samples per line period are enough to get a good RMS value. So the
minimum reading frequency per phase with WFORM should be 60*10=900Hz.
600Hz*3=1800Hz. So reading all 3 phases at 1.8KHz, should be enough.